fetch-configlet: make some variables local

From e.g. the Google Shell Style Guide [1]:

    Ensure that local variables are only seen inside a function and its
    children by using `local` when declaring them. This avoids polluting
    the global name space and inadvertently setting variables that may
    have significance outside the function.

    Declaration and assignment must be separate statements when the
    assignment value is provided by a command substitution; as the
    `local` builtin does not propagate the exit code from the command
    substitution.

[1] https://google.github.io/styleguide/shellguide.html#use-local-variables

exercism/configlet#691

bin/fetch-configlet

@@ -6,29 +6,6 @@
 
 set -eo pipefail
 
-readonly LATEST='https://api.github.com/repos/exercism/configlet/releases/latest'
-
-case "$(uname)" in
-  Darwin*)   os='mac'     ;;
-  Linux*)    os='linux'   ;;
-  Windows*)  os='windows' ;;
-  MINGW*)    os='windows' ;;
-  MSYS_NT-*) os='windows' ;;
-  *)         os='linux'   ;;
-esac
-
-case "${os}" in
-  windows*) ext='zip' ;;
-  *)        ext='tgz' ;;
-esac
-
-case "$(uname -m)" in
-  *64*)  arch='64bit' ;;
-  *686*) arch='32bit' ;;
-  *386*) arch='32bit' ;;
-  *)     arch='64bit' ;;
-esac
-
 curlopts=(
   --silent
   --show-error
@@ -41,15 +18,25 @@ if [[ -n "${GITHUB_TOKEN}" ]]; then
   curlopts+=(--header "authorization: Bearer ${GITHUB_TOKEN}")
 fi
 
-suffix="${os}-${arch}.${ext}"
-
 get_download_url() {
-  curl "${curlopts[@]}" --header 'Accept: application/vnd.github.v3+json' "${LATEST}" |
+  local os="$1"
+  local ext="$2"
+  local latest='https://api.github.com/repos/exercism/configlet/releases/latest'
+  local arch
+  case "$(uname -m)" in
+    *64*)  arch='64bit' ;;
+    *686*) arch='32bit' ;;
+    *386*) arch='32bit' ;;
+    *)     arch='64bit' ;;
+  esac
+  local suffix="${os}-${arch}.${ext}"
+  curl "${curlopts[@]}" --header 'Accept: application/vnd.github.v3+json' "${latest}" |
     grep "\"browser_download_url\": \".*/download/.*/configlet.*${suffix}\"$" |
     cut -d'"' -f4
 }
 
 main() {
+  local output_dir
   if [[ -d ./bin ]]; then
     output_dir="./bin"
   elif [[ $PWD == */bin ]]; then
@@ -59,9 +46,26 @@ main() {
     return 1
   fi
 
+  local os
+  case "$(uname)" in
+    Darwin*)   os='mac'     ;;
+    Linux*)    os='linux'   ;;
+    Windows*)  os='windows' ;;
+    MINGW*)    os='windows' ;;
+    MSYS_NT-*) os='windows' ;;
+    *)         os='linux'   ;;
+  esac
+
+  local ext
+  case "${os}" in
+    windows*) ext='zip' ;;
+    *)        ext='tgz' ;;
+  esac
+
   echo "Fetching configlet..." >&2
-  download_url="$(get_download_url)"
-  output_path="${output_dir}/latest-configlet.${ext}"
+  local download_url
+  download_url="$(get_download_url "${os}" "${ext}")"
+  local output_path="${output_dir}/latest-configlet.${ext}"
   curl "${curlopts[@]}" --output "${output_path}" "${download_url}"
 
   case "${ext}" in
@@ -71,12 +75,14 @@ main() {
 
   rm -f "${output_path}"
 
+  local executable_ext
   case "${os}" in
     windows*) executable_ext='.exe' ;;
     *)        executable_ext=''     ;;
   esac
 
-  configlet_path="${output_dir}/configlet${executable_ext}"
+  local configlet_path="${output_dir}/configlet${executable_ext}"
+  local configlet_version
   configlet_version="$(${configlet_path} --version)"
   echo "Downloaded configlet ${configlet_version} to ${configlet_path}"
 }